The present invention relates generally to roof structures and more particularly to the utilization of a relatively inexpensive "sacrificial" roofing membrane located over and above a roofing membrane, specifically a built-up roof, for at least temporarily protecting the latter from the elements.
In order to cover and protect a flat or substantially flat roof deck, conventional practice has heretofore dictated the utilization of insulation sheet material, for example perlite insulation board, a built-up roof and particulate material, specifically gravel, in that order. The build-up roof which extends over and above the deck and insulation material, protects both against weathering generally and the gravel protects the built-up roof against harmful rays of the sun, particularly the ultraviolet rays. This type of roof structure has for the most part been found to be satisfactory for its intended purpose. However, it does include one particular disadvantage. Specifically, while the built-up roof may in large part be protected against the ultraviolet rays of the sun by means of the gravel located over its top surface, it is not protected against the other elements in the ambient surroundings including for example the rain and snow. Moreover, unless care is taken to uniformly and completely cover the built-up roof with the gravel, the latter does not provide complete and uniform protection against the sun's harmful rays.
When there is a failure in a roof structure of the type described, particularly when this failure occurs after a number of years, it is caused by the built-up roof being directly exposed to the elements. One way to protect the built-up roof is described in U.S. Pat. No. 3,411,256 which issued to J. S. Best on Nov. 19, 1968. In this patent, a roof deck is covered with a water impermeable membrane, specifically a built-up roof, thermal insulating material and gravel, in that order. It will be noted that in this particular roof structure the location of the built-up roof relative to the roof insulation is just the opposite of that previously described above. As a result, the roof insulation, which is located over the built-up roof, reliably protects the latter against the elements and particularly against the ultraviolet rays of the sun and against thermal shock.
There is, however, one major drawback in the particular structure just described. Since the thermal insulation is itself exposed to the weather, it cannot be constructed of just any thermal insulating material. For example, it cannot be constructed of conventional perlite insulation board, but rather requires the utilization of a material which can withstand exposure to the elements over the life of the roof, which can be as long as or longer than twenty years. In the Best patent this insulation is described generally as comprising a plurality of closed cell water impermeable insulating members. Particular reference is made to cellular plastic foams of a closed cell configuration including styrene polymer foams, styrene-acrylonitrile copolymer foams, styrene-methylmethacrylate copolymer foams and other water impermeable materials available in cellular form.
While, as stated, the Best structure provides reliable protection to its built-up roof, there are three particular disadvantages in utilizing such a structure. First, the overall thermal efficiency of the foam insulation layer is reduced by precipitation or melting snow and ice running through the spaces between insulating members. Second, this type of structure can be used only where roof insulation is itself a necessary part of the overall structure. Where for example a roof structure does not require insulation, a system of the type described in the Best patent could not be used, unless of course one wanted to go to the expense of providing such insulation anyway. Third, this type of structure requires the utilization of a large amount of gravel, in the neighborhood of 1000 lbs/100 ft.sup.2 to 1500 lbs/100 ft.sup.2, in order to prevent flotation or blow-off of the closed cell insulation.
As will be seen hereinafter, the roof structure of the present invention eliminates these latter disadvantages of the Best structure while at the same time overcoming the previously discussed disadvantage of the more conventional roof structure described above.